Tài liệu Chapter 9: Monoprotic Acid-Base Equilibria pdf

Chapter 9:

Monoprotic Acid-Base Equilibria

Chapter 11:

Acid-Base Titrations

Example: Determination of Example: Determination of HCl

concentration by titration with concentration by titration with NaOH

I. Solutions and Indicators for Neutralization Titrations

A. Standard Solutions:

The standards solutions used as titrants for unknown weak

acids or bases are always strong bases or acids, respectively.

Standard titrant acids: dilute solutions of HCl, HClO

4, or H

2SO

4.

Standard titrant bases: dilute solutions of NaOH, KOH.

The primary standards may not be strong acids or bases (e.g.,

potassium acid phthalate, sodium oxalate, sodium

bicarbonate).

B. The Theory of Indicator Behavior

1. pH-sensitive dyes have long been used as indicators.

Normally, the basic form (In) on the dye has a color different from the acid

form, HIn:

HIn + H2O <====> H3O+ + In￾In + H2O <====> OH- + HIn+

[HIn]

[H O ][In ] Ka 3 + − = [Eq.1]

[In]

[OH ][HIn ] Kb − + = [Eq. 2]

We can see that both equilibrium constant expressions above can be written:

[In ]

Ka[HIn] [H O ] 3 − + = [Eq. 1’]

[In]

Ka[HIn ] [H O ] 3 + + = {Note: Kw =KaKb = [OH-][H+]} [Eq. 2’]

Therefore, the [H3O+] determines the ratio of the acid/conjugate base form of

the indicator.

To see the color of a particular form (acid or base) of the indicator, that

form must be present at tenfold higher concentration.